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Modem, an acronym for modulator/demodulator, is a device that allows one computer to "talk" with another over a standard telephone line. Modems act as a kind of interpreter between a computer and the telephone line. Computers transmit digital data, expressed as electrical impulses, whereas telephones transmit voice frequencies as analog signals. To transmit digital data, the sending modem must first modulate, or encode, a computer's digital signal into an analog signal that can travel over the phone line. The receiving modem must then demodulate, or decode, the analog signal back into a digital signal recognizable to a computer. A modem transmits data in bits per second (bps), with the fastest modems transmitting at 56K (kilobits per second). An internal modem is housed within the computer itself, while an external modern is a separate device connected to the computer by cable.

A variety of protocols (standards, rules) govern the conversion of data to and from digital and analog. These also govern error correction and data compression. Error correction is necessary to detect and correct data that may have become lost or garbled as the result of a poor telephone connection. Data compression speeds the data transfer by eliminating any redundant data sent between two modems, which the receiving modem then restores to its original form. Individual modems vary in the types of protocols they support, depending on such factors as manufacturer and age.

Communications software enables a modem to perform the many tasks necessary to complete a session of sending and receiving data. To initiate a modem session, the user issues the command appropriate to the software being used; the software then takes over and begins the complicated process of opening the session, transferring the data, and closing the session.

To open the session, the software dials the receiving modem and waits for an answering signal. Once the two modem have established a connection, they engage in a process called "handshaking": they exchange information about the types of protocols each uses, ultimately agreeing to use a set common to both. For example, if one modem supports a more recent set of protocols then does the other, the first will agree to use the earlier set so that each is sending data at the same rate, with error correction and data compression appropriate to those protocols. The handshaking process itself is governed by its own protocol.

In addition to transmitting and receiving data, the communications software may also automate other tasks for the user, such as dialing, answering, redialing, and logging onto an online service.


The functionality provided by a traditional dial-up modemthe ability to send and receive information electronicallyis also offered in other technologies that offer faster transmission speeds, although each is not without its disadvantages. Integrated Services Digital Network (ISDN), Asymmetric Digital Subscriber Lines (ADSL), and Digital Subscriber Lines (DSL) all use more capacity of the existing phone to provide services.

At 128K, ISDN is more than twice as fast as a dialup modem, but not nearly as fast as ADSL or DSL. ADSL can deliver data at 8 mbps, but is available only in selected urban areas. DSL transmits at a high rate of speed, but to ensure reliable service, the user must be located near the phone company's central office or outlying transmitters. In addition, a DSL connection is always "on" and thus makes a computer more vulnerable to attacks from hackers. To secure a DSL connection, a user must install either a software package called a firewall or a piece of hardware called a router. With either of these in place, the DSL connection cannot be detected by outsiders.

Cable modems do not use phone lines. Instead, they utilize the same line that provides cable TV services to consumers. Offered by cable television companies, cable modems are about 50 times faster than a dialup modem, but transmission speed is dependent on the number of subscribers using the service at the same time. Because the service uses a shared connection, its speed decreases as the number of users increases. Satellite, or wireless, services are faster than a 56K modem, but slower than a DSL. In addition, the initial satellite installation is expensive. However, for users in rural areas who do not have access to other services, wireless service may be a viable option.


As Bonnie Lund states in Business Communication That Really Works!, "the speed with which we can exchange documents has revolutionized business communications," which in turn has enabled business to be done "faster, cheaper, and more efficiently." Modems, along with the related technologies, facilitate this rapid transfer of information between colleagues or customers, regardless of their location. Communications that, in the past, may have taken several days or even weeks to complete, can now be accomplished in a fraction of the time. For example, during a typical work day, an employee could use a modem to facilitate sending an email message to a customer, transmitting a spreadsheet containing the annual budget to a manager for review, or downloading a file from the Internet. On a busy day this will take place dozens of times.

Lund also notes that "modems are changing the work style of corporate America" by enabling workers to telecommute or telework. As Amy Joyce reported in the Washington Post, citing data from the Telework Association and Council (ITAC), "the number of employed Americans who performed any kind of work from home, from as little as one day a year to full time, grew from 41.3 million in 2003 to 44.4 million in 2004, a 7.5 percent growth rate." A related development is the "distributed office," used in many small businesses involved in consulting, software development, publishing, and similar industries were members of the company work from home communicating by e-mail and using a common network server available to each by one of the faster modes of telecommunications.


"Gateway Enables Remote Site Management." Product News Network. 18 April 2006.

Joyce, Amy. "Getting the Job Done at Home: Telecommuting Can Save Money and Boost Productivity." Washington Post. 26 June 2005.

Lund, Bonnie. Business Communication That Really Works! Affinity Publishing, Inc. 1995.

Poor, Alfred. "Phone as Modem?." Computer Shopper. April 2006.

Rae-Dupree, Janet. "Surfing the Web at Warp Speed with Minimal Expense." U.S. News & World Report. 19 June 2000.

                             Hillstrom, Northern Lights

                              updated by Magee, ECDI

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modem [modulator/demodulator], an external device or internal electronic circuitry used to transmit and receive digital data over a communications line normally used for analog signals. A modem attached to a computer converts digital data to an analog signal that it uses to modulate a carrier frequency. This frequency is transmitted over a line, frequently as an audio signal over a telecommunications line, to another modem that converts it back into a copy of the original data.

Synchronous data transmission uses timing signals in the data stream along with transmitted bits of uniform duration and interval. This permits the receiving modem to ignore spurious signals that do not conform to the anticipated signal. Asynchronous data transmission relies instead on various error-correcting protocols. Although most modems are either of the synchronous or asynchronous variety, some employ both methods of communication. Wireless modems send or receive data as a radio signal. A fax modem enables a computer to send and receive transmissions to and from a fax machine (see facsimile) or another fax modem.

Modems were first used with teletype machines to send telegrams and cablegrams. Digital modems were developed from the need to transmit large amounts of data for North American air defense during the 1950s. The first commercial modem was introduced in 1962. Dennis C. Hayes invented the personal computer modem in 1977, marking the emergence of the online and Internet era. In the beginning modems were used primarily to communicate between data terminals and a host computer. Later the use of modems was extended to communicate between hosts in networks. This required modems that could transmit data faster, leading to the introduction of compression techniques to increase data rates and error detection and correction techniques to improve reliability. However, still faster transmission speeds were required.

A traditional modem, operating over traditional—mostly analog—phone lines, has a data transmission speed limit of about 56 kilobits per second. A specification for an Integrated Services Digital Network (ISDN), which allows wide-bandwith digital transmissions using the public switched telephone network, was introduced in 1984. A phone call can transfer 64 kilobits of digital data per second with ISDN and 128 kilobits with dual-channel ISDN. ISDN connections are used to provide a wide variety of digital services including digital voice telephone, fax, e-mail, digital video, and access to the Internet.

Faster still are the Digital Subscriber Line (DSL) protocol, introduced in the early 1990s, and the cable modem, introduced in the late 1990s. Each of these has a maximum data transfer rate of 1.5 megabits per second. DSL provides a broadband digital communications connection that operates over standard copper telephone wires. The connection requires a DSL modem, which splits transmissions into a lower band for ordinary telephone calls and an upper band for digital data. The drawback of DSL is that connected computers must be within a few miles of the closest transmitting station. A cable modem modulates and demodulates signals like a telephone modem but it transfers data much more quickly over cable lines—primarily fiber-optic or coaxial cable. Broadband over Power Lines (BPL) modems work similarly but utilize electrical lines to transfer data; BPL modems are plugged into electrical outlets. BPL modems may be used to access an Internet service provider over the local power lines, or they may use the wiring within a building to create a network for the computers there.

See also baud; code; modulation.

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modem Short for modulator and demodulator. A device that can convert a digital bit stream into an analog signal suitable for transmission over some analog communication channel (modulation), and can convert incoming analog signals back into digital signals (demodulation). Modems are used to connect digital devices across analog transmission lines. Most modems are designed to match specific national or international standards so that data communication equipment from one manufacturer can talk to that of another.

Modems can be packaged in many ways: as add-in cards or PC cards allowing personal computers to communicate over ordinary phone lines, as small external units, or as rack-mounted sets for large applications requiring many simultaneous connections. Modern modems, for attachment to a normal telephone line, can handle data at 56 000 bps, which is about the upper limit attainable over voice-frequency channels. Still faster modems for use in areas where telephone exchanges are equipped with broadband are available, allowing users to connect at over 500 kbps using ADSL or similar services.

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modem (modulator-demodulator) Electronic device for sending and receiving computer signals through a telephone system. The electrical pulses produced by a computer are fed into a modem, which uses the pulses to modulate a continuous tone (carrier) by a process called frequency modulation (FM). At the other end, another modem extracts the pulses (demodulation), so that they can be fed into a receiving computer. See also computer network; Internet

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modemahem, Belém, Clem, condemn, contemn, crème de la crème, em, gem, hem, Jem, LibDem, phlegm, pro tem, rem, Shem, stem, them •carpe diem, per diem •proem • idem • modem • diadem •mayhem • Bethlehem • ad hominem •ad valorem • brainstem •apophthegm (US apothegm)

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mo·dem / ˈmōdəm; ˈmōˌdem/ • n. a combined device for modulation and demodulation, for example, between the digital data of a computer and the analog signal of a telephone line. • v. [tr.] send (data) by modem.

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modem (ˈməʊdɛm) Computing modulator demodulator